The use of roller rocker arms and a valve mechanism of a hydraulic tappet in an overhead cam engine structure is increasingly widespread, compared with a valve mechanism of a mechanical tappet, it has the following advantages:
1) real-time compensation of valve clearance, not requiring a periodic manual adjustment;
2) reducing engine noise and shock;
3) lighter volume mass, smaller friction, smaller movement inertia meaning less energy loss, conducive to improving engine efficiency; and
4) longer service life of components and parts of the valve mechanism, and reduced maintenance cost.
The columnar hydraulic tappet has a can-shaped housing. In the housing, it is matched with a plunger that axially moves relatively to the housing, a high-pressure chamber for a hydraulic medium is formed between a lower end surface of the plunger and a bottom of the housing, and the high-pressure chamber can be sealed by means of a check valve opened towards the high-pressure chamber. The high-pressure chamber is internally provided with a reset spring which acts between the lower end surface of the plunger and the housing. The housing is provided with an oil hole for hydraulic medium circulation. The oil hole is communicated, at a radial inner side of the housing, with an oil feed hole of the plunger. An oil storage chamber for storing hydraulic medium is formed between the oil feed hole of the plunger and the check valve. An external cylindrical surface at the lower end of the plunger is matched with an internal cylindrical surface at the lower end of the housing to form a leak-down sealing region therebetween, which has a certain length and a certain clearance, so as to ensure the columnar hydraulic tappet has a certain leak-down feature.
When the engine stopped working for some time, the hydraulic tappet is pressurized at a lower position, a part of the hydraulic medium in the high-pressure chamber is discharged. When the engine is started, the hydraulic medium supplied by an oil pump needs some time to reach the hydraulic tappet. During this period, a valve clearance exists. The plunger of the hydraulic tappet moves upward under the action of the reset spring, so that a negative pressure is formed in the high-pressure chamber. The check valve is opened, and the hydraulic medium in the oil storage chamber is sucked into the high-pressure chamber. The hydraulic tappet is gradually restored to a working height, and the valve clearance disappears. If oil in the oil storage chamber is insufficient, air is sucked into the high-pressure chamber, and supporting stiffness of the hydraulic tappet is substantially reduced, which causes abnormal engine sound, increases noise, and accelerates wear of related parts.
A Chinese Patent Document CN102767405 discloses a hydraulic support element, as shown in FIG. 1. A plunger of this structure comprises an upper and a lower plunger bodies. A leak-down sealing region is formed between an external cylindrical surface of the lower plunger body and an internal cylindrical surface of the housing by means of fitting, while the upper plunger body mainly plays a role in guiding. A fit clearance between the external cylindrical surface of the upper plunger body and the internal cylindrical surface of the housing is slightly larger, which tends to slightly tilting in the working process. Therefore, in order to ensure that when the hydraulic tappet is compressed downward from the working height, the upper plunger body does not stagnate on an internal and round step surface of the housing. A guiding length of the upper plunger body in an inner hole of the housing is spacially limited. A height dimension of the lower plunger body shall not be too long. It shall ensure that when the hydraulic tappet is at the working height, the lower end surface of the upper plunger body is lower than the internal and round step surface at the lower end of the oil hole of the housing. Because the height dimension of the lower plunger body is limited, and the length of the leak-down sealing region is also limited, the clearance value of the leak-down sealing region cannot be too large, the requirements for cylindricity of the housing and the plunger are higher, which increases the processing difficulties. In addition, the hydraulic tappet of this structure has a grinding wheel groove with a certain width on the internal cylindrical surface of the housing thereof. Therefore, in the process when the hydraulic tappet is compressed from its working height to a minimum position, the length of the leak-down sealing region is gradually reduced, thereby causing inconsistency of leak-down time within a working stroke. In order to ensure enough sealing length, the grinding wheel groove of the housing of this structure can only be designed relatively narrowly. In an internal cylindrical shaping and grinding procedure, the front end of a grinding wheel is easy to wear because it is a dead hole grinding, it is possible that the diameter of an inner hole at the upper end of the grinding wheel groove, i.e., near the bottom of the inner hole, may become slightly smaller, which may cause clamping stagnation of the plunger and even function failure of the product lacking sufficient clearance. A split-type plunger structure needs to be processed respectively, thus the manufacturing cost is relatively high. Particular requirements for runout of the contact end face of two plunger bodies are very high. If the runout is out of tolerance, a lateral force may be generated, further possibly causing the plunger is clamped and stagnated in the housing, thereby affecting the use function of the product.